Blood gas analyzers typically include electrodes that measure the pH, the partial pressure of CO.sub.2 (PCO.sub.2), and the partial pressure of O.sub.2 (PO.sub.2) of a blood sample. Such electrodes generally are calibrated prior to use on a blood sample. Calibration of an electrode involves contacting the electrode with at least two standard solutions or gases having known concentrations of the substance being analyzed (e.g., CO.sub.2). The electrode provides an electrical response that is used to generate a calibration slope. The electrode then is contacted with the blood sample, generating further electrical response, and the calibration slope is used to convert this electrical response into the concentration of the substance in the blood sample, Electrodes in blood gas analyzers are calibrated periodically because the response provided by an electrode for a particular sample tends to drift (vary) with the passage of time.
Different types of standard solutions or gases have been used to calibrate CO.sub.2 and O.sub.2 ELECTRODES. One type of calibration standard is generated by bubbling a dry CO.sub.2 and O.sub.2 gas mixture through a humidifier. If the volume fraction of CO.sub.2 and O.sub.2 in the dry gas mixture are known, the PCO.sub.2 and PO.sub.2 in the humidified gas can be calculated as follows: EQU PCO.sub.2 =vCO.sub.2 .times.(P.sub.atm -47 mmHg) EQU PO.sub.2 =vO.sub.2 .times.(P.sub.atm -47 mmHg)
In the equations, the P.sub.atm is the atmosphere pressure (which is equal to 760 mmHg at standard condition), and 47 mmHg is the saturated water vapor pressure at 37.degree. C.
Gas tonometered aqueous solutions packaged in sealed containers also have been used as calibration standards for CO.sub.2 and O.sub.2 electrodes. The manufacturing environment for the gas tonometered solutions requires precise temperature and pressure control to ensure the accuracy of the calibration standards. It generally also is necessary to equilibrate these standards, external of a blood gas analyzer, to a known temperature before using them to calibrate the electrodes in the analyzer. This equilibrating process can be very time consuming depending upon the ambient temperature and the amount of standard solution in the container. It also requires complex constant temperature controlling hardware.
Other calibration standards that have been used with CO.sub.2 and O.sub.2 electrodes contain water, an organic solvent, and a CO.sub.2 or O.sub.2 complexing agent to vary O.sub.2 and CO.sub.2 solubility in the sample.